(1) Field of the Invention
The present invention relates to image display apparatuses and particularly to an image display apparatus using a current-driven luminescence element and to a method of driving the image display apparatus.
(2) Description of the Related Art
An image display apparatus using an organic electroluminescence (EL) element (an organic EL display apparatus) is known as an example of an image display apparatus using a current-driven luminescence element. An organic EL display apparatus using such a self-luminous organic EL element does not require a backlight necessary for a liquid-crystal display apparatus and is most suitable when a lower-profile apparatus is desired. Since there is no limit on a viewing angle of an organic EL display apparatus, organic EL apparatuses are expected as next-generation display apparatuses to be put to practical use. An organic EL element used in an organic EL display apparatus is different from a liquid-crystal cell which is controlled by a voltage applied to the liquid-crystal cell in that luminance of each luminescence element is controlled by a value of a current flowing through the luminescence element.
In an organic EL display apparatus, organic EL elements each constituting a pixel are generally disposed in a matrix. Known organic EL display apparatuses include passive matrix organic EL display apparatuses and active matrix organic EL display apparatuses.
A passive matrix organic EL display apparatus has an organic EL element provided at each of intersections of a plurality of row electrodes (scanning lines) and a plurality of column electrodes (data lines) and drives the organic EL elements by applying a voltage corresponding to a data signal between a selected one of the row electrodes and the plurality of column electrodes.
An active matrix organic EL display apparatus has a switching thin film transistor (TFT: Thin Film Transistor) provided at each of intersections of a plurality of row electrodes and a plurality of data lines, and a gate of a drive element is connected to each switching TFT. The active matrix organic EL display apparatus inputs a data signal from a signal line to the corresponding drive element by turning on the switching TFT through a selected one of the scanning lines. An organic EL element is driven by the drive element.
An active matrix organic EL display apparatus can cause organic EL elements to generate photons until a next scanning (selection) operation, unlike a passive matrix organic EL display apparatus whose organic EL elements connected to respective row electrodes (scanning line) generate photons only during a period when the scanning line is selected. Even an increase in duty ratio does not invite a reduction in luminance of the display. An active matrix organic EL display apparatus can thus be driven at a low voltage, which allows a reduction in power consumption.
However, an active matrix organic EL display apparatus suffers from the problem that even with the same data signal supplied, organic EL elements are different in luminance among pixels due to variation in characteristics of drive transistors, i.e., unevenness in luminance shows up.
To cope with the problem, a method for compensating for variation in characteristics of pixels is disclosed as a method for compensating for unevenness in luminance due to variation in characteristics of drive transistors (see, e.g., Japanese Unexamined Patent Application Publication No. 2008-033193 (hereinafter referred to as Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2007-310034 (hereinafter referred to as Patent Document 2).
A display apparatus disclosed in Patent Document 1 will be described below.
FIG. 13 is a diagram showing a configuration of a display apparatus 100 disclosed in Patent Document 1.
A pixel array unit 102 includes rows of scanning lines WSL, columns of signal lines DTL, a matrix of pixels 101, each of which is disposed at an intersection of one of the scanning lines WSL and one of the signal lines DTL, and power lines DSL which are disposed for respective rows of the pixels 101. Furthermore, the display apparatus 100 includes a main scanner 104 which sequentially supplies control signals to the scanning lines WSL in respective horizontal cycles (1H) and line-sequentially scans the pixels 101, one row at a time, a power supply scanner 105 which supplies a power supply voltage that is switched between a first voltage and a second voltage to the power lines DSL in step with the line-sequential scanning, and a signal selector 103 which switches between a signal voltage serving as a video signal and a reference voltage and supplies one of the voltages to the columns of signal lines during each horizontal period (1H) in step with the line-sequential scanning.
Each pixel 101 includes a luminescence element 3D, typified by, e.g., an organic EL element, a transistor 3A for sampling, a transistor 3B for driving, and a holding capacitor 3C.
In the transistor 3A for sampling, a gate is connected to the corresponding scanning line WSL, one of a source and a drain is connected to the corresponding signal line DTL, and the other is connected to a gate of the transistor 3B for driving.
In the transistor 3B for driving, one of a source and a drain is connected to the luminescence element 3D, and the other is connected to the corresponding power line DSL.
A cathode of the luminescence element 3D is connected to ground wiring 3H. The holding capacitor 3C is connected between the source and the gate of the transistor 3B for driving.
With the above-described configuration, the transistor 3A for sampling is brought into conduction in response to a control signal supplied from the scanning line WSL, samples a signal voltage supplied from the signal line DTL, and holds the signal voltage in the holding capacitor 3C. The transistor 3B for driving is supplied with a current from the power line DSL at a first potential and feeds a drive current through the luminescence element 3D according to the signal voltage held in the holding capacitor 3C.
During a time period when the power line DSL is at the first potential and the signal line DTL is at the reference voltage, the main scanner 104 outputs a control signal for bringing the transistor 3A for sampling into conduction and performs a threshold voltage correction operation for holding a voltage corresponding to a threshold voltage Vth of the transistor 3B for driving in the holding capacitor 3C.
Prior to the above-described threshold voltage correction operation, during a time period when the power line DSL is at a second potential and the signal line DTL is at the reference voltage, the main scanner 104 outputs a control signal to bring the transistor 3A for sampling into conduction, sets the gate of the transistor 3B for driving to the reference voltage, and sets the source to the second potential. Such an operation of resetting a potential of the gate and a potential of the source makes it possible to reliably perform the following threshold voltage correction operation.
As described above, the display apparatus 100 disclosed in Patent Document 1 performs a reset operation before a threshold voltage correction operation by supplying the power lines DSL with the second potential different from the first potential supplied during a normal operation. With this configuration, the display apparatus 100 disclosed in Patent Document 1 can implement a threshold voltage correction operation without adding transistors.